北师大版高一必修1教材：

My sister, Alison, and I sat down in front of the fire and wrote a letter to Father Christmas telling him about all the presents we wanted.

句中telling him about all the presents we wanted为现在分词短语，在句中作伴随状语。--------教辅原话。

个人认为，telling him about all the presents we wanted在这里解释为a letter的定语更为合适，望老师批评指正。